#include <CXdrFilterOutputStage.h>
class CXdrFilterOutputStage : public CFilterOutputStage
public:
CXdrFilterOutputStage();
virtual void open(std::string filename) ;
virtual void close() ;
virtual void DescribeEvent(std::vector<std::string> parameterNames,
std::vector<UInt_t> parameterIds);
virtual void operator()(CEvent& event);
virtual std::string type() const;
};
This concrete output stage uses XDR (EXternal Data Represntation) to write a system independent form of filter output data. See the section File Format below for a description of the file format.
CXdrFilterOutputStage();
Constructs an Xdr Filter.
virtual void open(std::string filename);
Called to open the file. This creates a
CXdrOutputStream
object that is open on the filename.
That object is used to perform action serializations
of primitive items to file.
virtual void close();
Closes the output file. The
CXdrOutputStream object
created in open
is destroyed.
virtual void DescribeEvent(std::vector<std::string> parameterNames, std::vector<UInt_t> parameterIds);
Called when the filter knows what parameters will be output
to the file. parameterNames
is a vector of the names of the parameters while
parameterIds are the ids
of the parameters that are actually in the SpecTcl
parameter dictionary.
A series of header records are
written via the CXdrOutputStream.
These records are described in
File Format below and describe the
set of parameters that will be written to the filter
file.
virtual void operator()(CEvent& event);
Called to write an event to the filter files. An event record is written to file. See File Format for the contents of this record.
virtual const std::string type();
Returns the string xdr to signify that this output stage produces XDR formatted data.
The file produed by CXdrOutputStage objects
consists of a set of records. Each record is identified by
a string record type. At present, two record types are defined:
header which contains a set of parameter names
and event which contains an event.
The file consists first of one or more header items followed by a sequence of event items. Items are written into fixed length 8192 byte blocks. Each header item is in a block by iteself. Several event items will be packed into a single block.
A header block consits of an XDR string with the value header. This is then followed by a count of strings to follow. The count is an xdr integer. The strings that follow are each the name of a parameter that is written to the filter file.
It is important to maintain the order in which the parameters appear as this will be needed to decode the following event items. There can be several header items. They cumulatively describe the set of parameters written to the file.
Event items each have a compressed event. The compression scheme takes advantage of the fact that in most cases, the occupancy of parameter space is sparse for an event.
Each event begins with a bitmask. The set bits in this bitmask determine the number of parameters that follow. Each element of the bitmask is an XDR integer and the number of bitmasks is determined by the number of 32 bit integers (XDR integers are 32 bits wide) required to provide one bit for each parameter.
Bitmasks are ordered by parameters as they appear in the header records. The first bitmask has a bit for each of the first 32 parameters and so on. Within each 32 bit bitmask, bits are assigned from low order to high order within the 32 parameter group.
Following the bitmasks are a set of XDR double precision values. Each of those values represents a parameter value for one of the parameters corresponding to the bits set in the bitmask. The first parameter is for the lowest order bit set in the first nonzero bitmask, and so on.
Suppose, for example, we have an event with 48 parameters. This requires two bitmasks. All bits in the first bitmask are used and the lower 16 bits in the second one are used. Suppose the value of the first bitmask is 0x0000000a and that of the second one is 0x00000001. Three parameter values will follow.
If we number the parameters from zero; The first value will be for parameter 1, the second for parameter 2 and the last for parameter 32.